This application is based on and claims benefit of Japanese Application Ser. No. 10-252660, file Sep. 7, 1998, to which a claim of priority is made under 35 U.S.C. xc2xa7119.
The present invention relates to an optical amplifier, especially to an optical amplifier that is used as a multistage relay of a wavelength division multiplexing transmission system.
Wavelength division multiplexing transmission (WDM transmission) in practical use provides high capacity communication. In a typical WDM transmission system, gain difference in each wavelength constituting a transmission wave becomes a major issue in a multistage optical amplifier relay. Accordingly, a low gain tilt is required to achieve high capacity communication in a WDM transmission system.
In a conventional gain equalization method, the transmission system is optimized by adjusting each optical amplifier to achieve overall performance. Alternatively, a fixed gain equalizer is used with each optical amplifier being used as an optical relay, as shown in FIG. 8.
In the optical relay of FIG. 8, a wavelength synthesizing coupler 1 synthesizes excited light generated by an excited light source 6 and an input optical signal. A rare-earth element doped fiber 2 that has received the synthesized light optically amplifies an input signal by means of excited light in the synthesized light. An isolator 3 prevents oscillation associated with the optical amplification. An optical branch 4 receives an output from the isolator 3 and generates an output that is converted into a current by a light receiving element 9. An automatic gain control circuit 8 controls an output level of the excited light source 6. Another output from the optical branch 4 is typically kept constant, based on the current generated by the light receiving element 9. Outputs from the optical branch 4 are input to fixed gain equalizer 18, and a gain difference of each wavelength is equalized and output.
Also, a method of obtaining optimum gain equalization in a transmission system is disclosed in JP-A-223136/1996, entitled xe2x80x9cGain Equalization Method In Optical Amplification Relay Techniquexe2x80x9d by Mr. Fukutoku, et al. A method of improving a gain difference by wavelengths of an optical amplifier is disclosed in JP-A-43647/1997, entitled xe2x80x9cGain Equalizer and Optical Amplifierxe2x80x9d by Mr. Domon.
With regard to the above-mentioned gain equalization method, the optimum design for every transmission system varies in practice. The difference between the design and the practical implementation raises serious issues because the method is not general. The fixed gain equalizer method described above has a drawback in that a gain difference in each wavelength is dependent on variations in the gain of the optical amplifier due to temperature or change in input level of an input signal during wavelength division multiplexing transmission. Furthermore, the method in JP-A-223136/1996 does not lend itself to application to individual relays. In addition, the method in JP-A-43647/1997 realizes gain equalization by mechanically bending a fiber thereby being dependent on changes in temperature characteristics and provoking reliability concerns.
The objective of the present invention is to eliminate a gain difference in each wavelength based on variable operating environments including temperature and input level changes, and to provide an optical amplifier that can be used with each. relay.
An optical amplifier according to an embodiment of the present invention comprises:
an excited light source for generating excited light;
a wavelength synthesizing coupler for synthesizing an input optical signal and excited light generated by the above-described excited light source and outputting the resulting signal;
a rare-earth doped fiber for receiving an output from the above-described wavelength synthesizing coupler, and amplifying an input optical signal in the above-described output, which is excited by excited light in the above-described output;
an isolator connected to the above-described rare-earth doped fiber, for preventing oscillation associated with optical amplification of above-described fiber;
an optical branch for receiving the above-described amplified input optical signal which is output from the above-described isolator, and dividing it into outputs;
a first light receiving element for receiving a first output light from the above-described optical branch, and converting it into a current;
a an automatic gain control circuit for controlling an output level of said excited light source so as to make a level of a second output light from the above-described optical branch constant, based on current information output from the first light receiving element;
a second light receiving element for receiving excited light output from the above-described excited light source, and converting it into a current;
one or more than one variable gain equalizers for equalizing a gain difference of each wavelength of input light, and outputting it; and
one or more than one variable gain equalization control means for controlling the above-described variable gain equalizers based on current information output from the second light receiving element so that a gain difference in each wavelength of light which is output from the above-described variable gain equalizers becomes constant.
The above-described variable gain equalization control means may have:
a current/voltage conversion circuit for receiving a current generated by the above-described second light receiving element, and converting it into a voltage;
a voltage control frequency generator for generating an alternating current having a frequency proportional to a level of a voltage generated by the above-described current/voltage conversion circuit;
an amplifier for amplifying an amplitude of the alternating current generated by the above-described voltage control frequency generator;
a control circuit for controlling the above-described amplifier so that an amplitude of the alternating current output by the above-described amplifier becomes to be a required value; and
a drive circuit for receiving an alternating current output by the above-described amplifier, and driving the above-described variable gain equalizers.
The above-described variable gain equalizers may be inserted after the above-described optical branch.
The above-described variable gain equalizers may be inserted before the above-described wavelength synthesizing coupler.
The above-described variable gain equalizers may include one or more inserted after above-described optical branch and one or more before the above-described wavelength synthesizing coupler.
The above-described variable gain equalizers may be controlled by one of the variable gain equalization control means.